Neuropsychologia.
Vol. 18 pp. 41 to 47.
Q Pergamon Press Ltd. 1980. Printed in Great Britain.
HEMISPHERIC
ASYMMETRY FOR AUDITORY
PERCEPTION OF TEMPORAL ORDER
LAURAINMILLS and GARY B.
Department
ROLLMAN*
of Psychology, University of Western Ontario, London, Ontario, Canada
(Received 13 June 1979)
Abstract-Two
psychophysical methods were employed to examine the role of the left cerebral
hemisphere for the auditory discrimination of temporal order. Subjects were asked to report
either the order or the simultaneity of two clicks when each was presented to a different ear.
The results showed that the threshold for temporal order was smaller when the right-ear click
preceded the left-ear click compared to the opposite order of presentation. These results were
discussed in relation to an hypothesis which suggests that the left hemisphere is the location
for temporal processing in this task.
THE PURPOSE
of the present experiment was to investigate brain asymmetry for the auditory
discrimination of temporal order in a sample of normal subjects. EFRON[l] showed that the
visual and tactile discrimination of temporal order occurred most efficiently within the left
cerebral hemisphere, but did not investigate temporal order perception in the auditory
modality. A demonstration of hemispheric asymmetry for decisions of auditory temporal
order would have implications for the description of information transmission in the
auditory system. As well, it would provide an important addition to the collection of studies
which attribute superiority of temporal function to one hemisphere [2-71.
METHOD
Subjects
Two independent samples of 10 right-handed college students, within an age range of 18-25 yr, participated in threshold determinations using the method of limits or the methodof constant stimuli. Their pure-tone
thresholds for the left and right ears were determined with the Bekesy audiometer. The resulting audiograms
showed that no subject had a difference in threshold of more than 5 dB between ears for frequencies spanning 750 Hz-4000 Hz. Handedness was assessed with a questionnaire consisting of six items. The skills rep:
resented were those found by ANNETT [8] to be actions consistently performed with the preferred hand.
Apparatus
The output of a Tektronix waveform generator was used to trigger two Tektronix pulse generators, each
of which produced a square wave pulse of I msec duration which the subject heard through stereophonic
headphones. Each pulse could be delayed relative to the other by means of a control on the pulse generator.
A Grason-Stadler white-noise generator was used to provide a background for the click pair. Calibration of
the pulse characteristics and the delay was performed immediately prior to an experimental session using a
Hewlett-Packard oscilloscope.
*This research was supported
Rollman.
by Grant AO-392 from the National Research Council, awarded to G. B.
41
42
LAURAINMILLS and GARY R. ROLLMAN
Procedwe
Pairs of successive clicks were presented to the subjects such that on each trial one member of the pair was
delivered to the left ear and the other member of the pair was delivered to the right ear. The interval of time
separating the onsets of the paired clicks was varied to include intervals at which their temporal order could
be resolved easily as well as intervals at which the temporal distance between stimuli was too small to allow a
ready discrimination of temporal order. In the latter case, subjects were instructed to call the stimuli “simultaneous”, as in EFRON’S[I] procedure, indicating that the two clicks were heard as occurring together in
time. This judgment necessarily included the range of successiveness for which two stimuli may be identified
even though their order cannot be. When subjects made successive-order judgments, they were instructed to
report the ear which received the first click of the pair.
The method of limits and the method of constant stimuli were used to obtained thresholds for theperception of successive order. In both methods, the trials with one ear leading were presented in a blocked sequence.
This paradigm was used to minimize effects due to attentional biases such as the law of prior entry would
predict [9].
Me/hod of limifs. Each subject received a total of IO descending runs presented in a prearranged random
sequence; five with the click to the left ear leading and five with the click to the right ear leading. The initial
separation between clicks in a run varied from 80 to 100 msec with a step size of 5 msec. A background of
white noise was superimposed in both ears during presentation of the click pairs to facilitate the use of
crossed auditory pathways. AITKEN[IO] measured right and left hand reaction time to monaural tones with
and without contralateral noise. He reported that crossed reaction time (right ear-left hand) was significantly
longer than uncrossed reaction time (left ear-left hand) in the presence of white noise, but obtained no
difference between conditions in the absence of contralateral noise. He concluded that the presence of white
noise favored the transmission of stimulus information via contralateral pathways. The intensity level of the
noise was set at a value estimated by the subject to be equal to the click which was 70 dB SL.
A run was terminated when the subject reported simultaneity for two successive trials and a trial was
repeated as many times as necessary for the subject to make a judgment. Two practice runs were given to
each subject prior to the experimental ones. The position of the headphones was reversed for one-half of the
subjects to control for any asymmetries in the audio channels.
Merhod ofcomfanf
stimuli. A click was delivered to the left or the right ear and, after a randomly-chosen
interval, the other ear received a click. Five intervals were used (IO, 20, 30,40 and 50 msec) and each interval
was presented 20 times in a right-first condition and 20 times in a left-first condition. A total of 240 trials
were given in the experiment, with 20 practice trials and 100 test trials per day. One half of the subjects
received the right-first trials on their initial day.
The instructions given to each subject were similar to those for the method of limits except that the subject
made a manual response with the right hand to indicate successive order or simultaneity, prior to orally
reporting which ear received the first click in the case of a successive-order judgment. It was assumed that the
use of the right hand did not influence measurement of the threshold since the obtained value was based on
the time interval between the first and the second click, and the response occurred after the second click.
RESULTS
Method of limits
The mean value of the interval within which together judgments occurred was calculated
for runs in which the right-ear click was delivered first and for runs in which the left-ear
click was delivered first. The mean threshold of temporal order for the right-first runs was
54.9 msec and for the left-first runs was 60.6 msec yielding a difference of 5.7 msec between.
conditions. A matched pairs t-test on these data indicated the ear difference to be significant
at the 0.01 level [t(9) = 4.191. As shown in Table I, a lower threshold of temporal order in
right-first runs was obtained for each of the 10 subjects tested.
Method of constant stimuli
In order to eliminate attentional asymmetries, left-first or right-first presentations
occurred
in blocks. Subjects, who were instructed to describe the perceived relationship between the
clicks, seemed to successfully heed those instructions
since reports at all inter-click intervals
incladed judgements
of “simultaneous”,
“left first” and “right first”. The results indicate
that the percentage of correct responses showed a gradual increase as temporal separation
ASYMMETRY
FOR AUDITORY
TEMPORAL
43
ORDER
Table 1. Mean threshold values tmsec) for individual subjects following right-first and left-first conditions
of stimulus presentation. Different groups of subjects were tested by the two methods
Method of limits
Left-first
Right-first
Mean
Standard deviation
16
83
65
13
63
54
59
39
72
82
60.6
19.42
14
14
61
57
56
48
57
35
70
77
54.9
18.01
Method of constant stimuli
Left-first
Right-first
48.0
35.6
27.9
42.0
40.0
23.8
46.2
28.6
30.0
32.0
35.3
7.95
36.1
40.0
25.0
35.0
36.1
25.0
43.3
25.4
25.1
28.8
32.2
6.61
was lengthened. At the briefest intervals, nearly all reports were “simultaneous”,
whereas
at the 50 msec separation, subjects reliably described the correct order.
The temporal gap for which subjects were correct on half of the trials approximates
the
threshold for successiveness under the conditions
of this experiment (for a separation of
A longer separation,
however, is
24 msec. 50% of the judgments
were “simultaneous”).
needed for the subjects to consistently report the correct temporal order. By definition they
could do this faultlessly when the percentage correct was 100. An estimation of the threshold
for temporal order was obtained by taking the point at which correct judgments were made
on 75 % of the trials.
The value at which a correct order judgment
was obtained on 75% of the trials was
derived for each subject using linear interpolation.
The mean threshold across subjects was
35.3 msec for left-first trials and 32.2 msec for right-first trials. The difference of 3.1 msec
was significant at the 0.05 level of confidence [?(9) = 2.42063. The individual
thresholds
shown in Table 1 indicate that eight of the ten subjects tested showed a lower threshold for
right-first trials.
The smaller mean threshold estimate obtained with this second method may be due to
the different psychophysical
technique as well as the greater number of trials delivered.
However, it is the relative difference between left- and right-ear thresholds, rather than the
absolute value of the threshold, which is pertinent to the hypothesis of the experiment and
the two techniques provided similar estimates of this difference.
DISCUSSION
The results of the present study demonstrate
that the threshold for temporal order is
lower when the right-ear stimulus precedes the left-ear stimulus compared to the opposite
order of presentation.
These findings are in agreement with those of EFRON [I] who found a
lower temporal-order
threshold for visual and cutaneous stimuli when the right-side stimulus
was delivered first. Efron’s interpretation
of these data suggested that the comparison
of
temporal order and the decision of simultaneity occurred within the left cerebral hemisphere.
LAURAIN MILLS and GARY B. ROLLMAN
44
He assumed that information
about each visual or cutaneous stimulus was initially directed
to the contralateral
hemisphere and primarily used contralateral
sensory pathways. Consequently, information
concerning the left-side stimulus required more time to reach the left
hemisphere
than was required for the right-side stimulus.
The higher-temporal
order
threshold which was found for the left-first condition included this additional transmission
time, assumed to rehect the callosal transfer of temporal information
from the right to the
left hemisphere.
In the auditory modality, the same rationale may be developed to account for the rhreshhold difference providing
the assumption
of contralateral
transmission
is warranted.
Physiological
data [II, 121 have indicated that the contralateral
system is functionally
superior to the ipsilateral during both monaural and dichotic stimulation,
although some
ipsilateral contribution
remains. The literature on ear differences in dichotic and monaural
tasks [I 3-l 51 is based upon the assumption of superiority of contralateral
pathways coupled
with an asymmetry in the central processing of auditory inputs.
The concept of a timing mechanism based in the left hemisphere leads to a model, shown
in Fig. I, specifying the relationship between psychophysical
temporal order thresholds and
neural events. The behavioral data can be used to determine the interhemispheric
transfer
time, which equals one-half of the difference between thresholds obtained for the left-first
and right-first conditions.
Left clrck first
Th
Left Ear
L&t f+nWPrWe
Time
(msec)
thresWd for
temeemt order (Th)
=m+t+T-m
= 10’4.50-10
=54msec
Obtaned
Rfght Ear
j-m.
t-
j+m
T
t
.:
64
54
14
0
Rlqht click fIr5t
Left Ear
Rlgnt Ear
Left f+emlSphere
Time fmsec)
n*
Th
i-met-
j-m-0
t
10
Thleft-f,rst - Thr’,ght-f,r-st =
T
6o
46
m+t+T-m-
[m+T-fm+t)]
Obtafred dlffef-ence 1-1 temqcml thresnoldr
the lntert-emlsqherfc tmnsfer tome
Obtomed thresrwki for
temecml cuder (Th)
= m +T -(met)
= 10.50 -0o.4)
= 46msec
=2t
equals twue
FIG. I. Schematic
representation
of the rationale
for the experimental
hypothesis.
If determination of successive
order is based upon processes within the left hemisphere,
it will take ,rz msec
for a right-ear input to be transmitted
to that hemisphere,
but it will take )II plus an additional
t msec (interhemispheric
transfer time) for a left-ear input to reach there. An internal timing
process begins upon arrival of the first click at the left hemisphere,
where neural events must
be separated
by Tmsec in order to discriminate
temporal order. The psychophysical
threshold
for temporal
order (7X) reflects both the central threshold
(T) and the transmission
times On
2nd t). In order to illustrate
the derivation
tn = IO mscc, t = 4 msec, and T = 50 msec.
of thresholds,
the above
example
assumes
tha‘t
ASYMMETRY
FOR AUDITORY
TEMPORAL
ORDER
45
In the present study, I8 of the 20 subjects tested showed a lower threshold when the rightear stimulus was delivered first. This is in accord with the assumption that the information
was carried on the contralateral
pathways and that the locus of processing was in the left
hemisphere. For them, the transfer of left-ear information
needed for the temporal discrimination is hypothesized
as occurring from the right hemisphere to the site of temporal
analysis in the left hemisphere. However, for the two subjects who obtained a lower threshhold for the left-first condition, the determination
of temporal order may have occurred in
the right hemisphere and for these individuals the transfer may have occurred from the left
hemisphere to the right hemisphere. Therefore, to consider one-half of the difference in
thresholds as an estimate of transfer time, it would be appropriate
to base the estimate on
the absolute value of the threshold difference. When this transformation
is made, the value
of the mean threshold difference from the method of constant stimuli is 4.4 msec and the
estimate of transfer time is 2.2 msec. The estimate of transfer time from the method of limits
is 2.8 msec. Both these values are consistent with previous estimates of interhemispheric
transfer time reported for auditory, visual and cutaneous stimulation
[I, 14, 171.
An alternatrve interpretation
of the effect described here could be considered. When the
left ear received the first click, the right hemisphere may have performed the task, but less
efhciently (i.e. less accurately or more slowly) than the left hemisphere. If this were the case,
then the ear effect might be based upon two kinds of temporal processes, differing in
accuracy or latency, rather than providing the basis for an estimate of internemispheric
transfer time. A discussion of some clinical data may aid in evaluating this notion.
Previous studies [I, $7, 17, 181 of right and left brain-damaged
subjects, employing a task
similar to that used in the present experiment, found hemispheric differences in successiveorder thresholds ranging from IO msec to 354 msec. with damage to the left hemisphere
producing higher thresholds. Clearly, such differences are in excesss of the 5.7 and 3.2 msec
mean threshold differences here reported. These clinical results suggest tnat right hemisphere
processes for this task are much slower than would be required to yield the effect described
here. While conceivable that such a very small hemispheric difference in timing processes
could exist in normal subjects, it seems more reasonable to consider the tigures obtained in
the present experiments as values from which an estimate of callosal transfer trme may be
derived.
The present results suggest that the left hemisphere is specialized for judgments of temporal order, a finding which supports the more general notion of left hemisphere superiority
for temporal processing. Although there are reports in the literature which identify the right
hemisphere as having a role in some kinds of temporal discriminations,
the left hemisphere
seems to be prepotent
in fine temporal analysis. MILLS and ROLLMAN [6] showed lefthemisphere superiority for processing information
relating to stimulus duration when the
durations were less than a critical value. Above that value, the right hemisphere’s participation appeared to be equivalent to that of the left.
MURI’HY and VENABLES[IY, 201, however. reported a left-ear advantage (sampling righthemisphere function) for discrimination
of one versus two clicks when they presented subjects with paired clicks, separated by O-100 msec, to the right or left ear. Although their
findings could appear LO suggest right--hemispheric
specialization
in processing temporal
intervals as small as 2 msec. data on temporal acuity from both normal and clinical samples
[2l-241 provide a more probable interpretation.
These studies have demonstrated
that while
the temporal order of stimuli separated by intervals of IO msec or less may be discriminated,
the basis for the discrimination
appears to be loudness or pitch cues rather than temporal
LAURAINMILLSand GARY B. ROLLMAN
46
cues per se. In
fact,
MURPHY and VENABLES’ [19, 201 subjects reported
facilitate their performance.
Accordingly,
using pitch cues to
their results might,
in fact, represent right-hemisphere involvement in tasks which were not essentially temporal.
The recognition of melodic line is another task for which right hemisphere specialization
has been proposed, since dichotic listening techniques with normal and brain-damaged
subjects have yielded left-ear advantages [25, 261. Although one component of melodic line
is the temporal patterning, subjects could employ non-temporal strategies in the recognition
task. For example, requiring a listener to adopt an experimentally induced strategy (analytic
vs nonanalytic) when making his identification may result in either left or right ear advantages [27].
The extent of right-hemisphere participation in temporal tasks remains ambiguous, in
part, because definitions of temporal tasks have often been unclear. A variety of “temporal
discriminations” may be identified [23, 281, but the perceptual cues underlying them are not
well known. Melodic recognition and MURPHY and VENABLES’ [ 19, 201 data reflect performance on nominal temporal tasks in which the discrimination of a stimulus varying in time is,
in fact, based upon non-temporal cues. To consider such tasks as representing temporal
analysis lends confusion to the hemispheric delineation’ of temporal function.
A meaningful description of the hemispheres’ respective roles should be based on the
comparison of performance in functional tasks alone, that is, tasks in which temporal
information per se provides perceptual cues for the discrimination. It is suggested that the
present experiment is an example of such a functional task. The temporal separation at
which a correct order judgment was made was beyond the range associated with peripheral
interaction of the stimuli, so that qualitative differences in the composite sound could not
have been the perceptual cues. Instead, the decision of temporal order had to be based upon
an actual temporal cue: the arrival times of the representations of the two stimuli at some
central location in the brain. Empirical analysis which distinguishes functional from nominal
temporal tasks is of first importance in beginning to understand
hemispheric
specialization
for temporal processing.
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